This invention relates to the recovery of uranium from subterranean ore deposits and more particularly to the stabilization of clays in such subterranean deposits during in-situ leaching or restoration operations.
In the in-situ leaching of subterranean uranium ore deposits, a lixiviant containing a leaching agent is introduced into the subterranean deposit through a suitable injection system. The lixiviant solubilizes uranium values as it traverses the ore body. The pregnant lixiviant is then withdrawn from the ore body through a production system and treated to recover the uranium therefrom by any suitable technique such as adsorption and elution employing an ion exchange resin and subsequent precipitation.
The lixiviants employed in the in-situ leaching operations may generally be classified as containing a carbonate or an acidic leaching agent. In acid leaching, the most commonly employed acid is sulfuric acid added in an amount to provide a pH in the lixiviant of about 2 or less. Carbonate lixiviants contain carbonate or bicarbonate ions or mixtures thereof which function to complex the uranium in the form of water-soluble uranyl carbonate ions. The carbonate lixiviants may be formulated by the addition of alkali metal or ammonium carbonates and/or bicarbonates or by the addition of carbon dioxide together, in most cases, with an alkaline agent, e.g. sodium hydroxide or ammonia, to control the pH.
Many subterranean uranium deposits contain substantial quantities of clays which base exchange with the alkali metal or ammonium ions present in the lixiviant. Where the lixiviant contains an ammonium carbonate and/or bicarbonate leaching agent, the ammonium ions adsorbed by the clays are a potential source of contamination to fresh ground waters. Thus, it is a conventional practice to treat the formation at the conclusion of the leaching operation to restore it to an environmentally acceptable state. For example, U.S. Pat. No. 4,079,783 to Snavely et al. discloses a process for removing uranium ions from a subterranean formation by the injection of a restoration fluid comprising an aqueous solution of a strong base such as sodium or calcium hydroxide. The ammonium ions are desorbed from the clay surfaces by base exchange with the sodium or calcium ions and are partially converted to ammonia which is not subject to adsorption and thus is produced to the surface. In another procedure disclosed in U.S. Pat. No. 4,162,707 to Yan, a halogenated restoration fluid is injected into the formation. The restoration fluid may take the form of chlorinated water or an aqueous solution of a hypochlorite such as sodium hypochlorite. The ammonium ions are desorbed from the clay surfaces and the ammonia formed in equilibrium with the ammonium ions reacts in the chlorinated water to form trichloroamine.
In many cases, the clays found in the subterranean formation present problems not only from the standpoint of providing adsorption sites but also because they are unstable in the presence of the injected aqueous solution, that is, they become hydrated and through one or more of a number of mechanisms tend to plug the formation, thus reducing its permeability. A process for avoiding such permeability reduction during a restoration procedure is disclosed in U.S. Pat. No. 4,114,693 to Foster et al. In this process, the restoration fluid contains a strong base as described in the aforementioned patent to Snavely et al. and in addition contains an effective amount of a monovalent salt such as sodium chloride. The salt functions to retard clay swelling in the presence of a restoration fluid, thus avoiding a decrease in permeability during the restoration operation.
As noted in U.S. Pat. No. 4,134,618 to Kossack, in-situ leaching operations may result in contaminants other than ammonium ions being left in the formation. Thus, at the conclusion of a leaching operation employing either an acidic or alkaline leaching solution, the formation may contain unacceptable amounts of water-soluble contaminants such as sulfates and oxidized uranium and molybdenum values. The patent to Kossack proposes a restoration process which involves the cycling of water low in total dissolved solids through the formation.